Fuel cell devices are becoming general in fulfilling different kind of electricity production needs. Fuel cell devices are electrochemical devices supplied with reactants for producing electrical energy, which enable production of electricity with a high duty ratio in an environmentally friendly process. Fuel cell technology is considered as one of the most promising future energy production methods.
FIG. 1 illustrates an exemplary fuel cell in accordance with a known implementation. As shown in FIG. 1, the fuel cell includes an anode side 100 and a cathode side 102 and an electrolyte material 104 between them. The reactants fed to the fuel cell devices undergo a process in which electrical energy and heat are produced as a result of an exothermal reaction.
In known solid oxide fuel cells (SOFCs) oxygen 106 is fed to the cathode side 102 and it is reduced to a negative oxygen ion by receiving electrons from the cathode. The negative oxygen ion goes through the electrolyte material 104 to the anode side 100 where it reacts with the used fuel 108 producing water and carbon dioxide (CO2). Between the anode and the cathode is an external electric circuit 111 for transferring electrons e− to the cathode. The external electric circuit 111 includes a load 110.
FIG. 2 illustrates an exemplary SOFC device in accordance with another known implementation. As shown in FIG. 2, the SOFC device can utilize as fuel, for example, natural gas, bio gas, methanol or other compounds containing hydrocarbons. The SOFC device in FIG. 2 includes planar-like fuel cells in stack formation 103 (SOFC stack). Each fuel cell includes an anode 100 and a cathode 102 as presented in FIG. 1. A portion of the used fuel is recirculated in feedback arrangement 109 through each anode.
The SOFC device of FIG. 2 includes a fuel heat exchanger 105 and a reformer 107. Heat exchangers 105 are used for controlling thermal conditions in fuel cell process and therefore, more than one can be located in different locations of the SOFC device. The extra thermal energy in circulating gas can be recovered in the heat exchanger 105 to be utilized in the SOFC device or outside in a heat recovering unit. The heat recovering heat exchanger can thus be located in different locations as shown in FIG. 2. The reformer is a device that converts the fuel, for example, natural gas, to a composition suitable for fuel cells, to a composition containing, for example, half hydrogen and other half methane, carbon dioxide, and inert gases. The reformer is not, however, necessary in all fuel cell implementations, but untreated fuel can also be fed directly to the fuel cells 103.
By using measurement means 115 (such as a fuel flow meter, current meter, temperature meter or any other suitable meter as desired) measurements of the anode recirculating gas can be carried out for the operation of the SOFC device. A portion (e.g., part) of the gas used at anodes 100 (FIG. 1) of the fuel cells 103 is recirculated through anodes in feedback arrangement 109 and thus in FIG. 2 is presented diagrammatically also as the other part of the gas is exhausted 114 from the anodes 100.
Known fuel cell devices produce electrical energy in the form of direct current having a low voltage level. The voltage level can be raised by combining several fuel cells or combinations of fuel cells to form a serial connection such as for example a stacked formation. Current-voltage characteristics of the fuel cells depend on for example reactant compositions, mass flow, temperature and pressure. Electrochemical reactions in the fuel cell can react quickly to fluctuations in the fuel cell load. However, the response capacity of reactants input system can be slower, such as response times of seconds or even minutes. When trying to obtain more efficiency out of fuel cells than the prevailing input of reactants allows, a weakening of fuel cell voltages can result, and even an irreversible deterioration of fuel cells is possible. In addition, load changes can cause rapid temperature changes in the fuel cell, which in high temperature fuel cells can cause harmful thermomechanical stress, resulting in significant reduction of performance and life time of fuel cells. Thus, fuel cell systems should be designed so that the load of each fuel cell is kept as constant as possible and a possible change in the load is tried to be carried out as controllable as possible.
When the fuel cells are used to obtain independent variable AC loads, or to supply power to a distribution network, a DC-AC converter can be used to convert DC power to AC power. There can also be a need for DC-DC converters to raise DC voltage obtained from the fuel cells to a level that is suitable for DC-AC converter. However, due to the highly limited compatibility and capacity of the fuel cells to respond to changes in load, known fuel cell implementations, such as high temperature fuel cell implementations, apply badly as power sources to feed independent variable AC loads or to feed variable power to the distribution network. A known way to try to fix said problem is the use of an energy buffer, which consists, for example, of lead acid batteries. The function of the energy buffer is to feed or consume power in rapidly changing conditions so that the load variation of the fuel cell can be controlled. In large fuel cell systems, for example, disadvantages of known implementations become more serious due to high cost, large size, heavy weight, and limited effectiveness. In electrical network coupled applications, another known implementation can maintain a constant fuel cell load for a current controlled transform in feeding power to the network. The control based on current controlled transform may not be suitable in a network independent operation, and thus it cannot be used as an emergency power source for critical AC loads inside or outside the fuel cell system.
High temperature fuel cell systems can specify a major heat energy amount for heating systems up to operating temperatures. From this follows that start-up times can be up to tens of hours in length. Wide temperature alternations in shut down and start up sequences expose the fuel cells and related system components to even excessive thermomechanical stress. Thus, the high-temperature fuel cell systems must be designed to operate continuously for as long time periods as possible, for even thousands of hours, without any shut downs. To achieve this objective the system should be designed to fulfil high reliability as well as to minimize such external factors, which might shut down the system or might drive the system to harmful operation conditions. Current controlled converters have faced issues in fuel cell applications as they are unable to protect the fuel cells from sudden changes in load, arising from different network disruptions such as power failures, voltage dips, or transients.
Heat management of the fuel cell stacks is one of the key functions of the balance of plant (BoP) equipment in a high temperature fuel cell system. The heat balance of the fuel cell stacks is affected by many mechanisms including internal reforming, fuel cell reactions, heat transport by flow of reactants and direct heat exchange with the surrounding structures. Exemplary methods for the control of the temperature balance include adjustment of the internal reforming rate and adjustment of air flow and cathode inlet temperature. The temperature control of stacks involves the temperature control of a significant amount of thermal mass in the stacks and related structures, hence introducing long response times to changes in operating parameters. These long response times, along with similar long response times in other parts of the BoP control limit the capability of the fuel cell system to respond to rapid load changes.
In known systems designed for stand-alone operation, power balance of the system should be controlled to match the demand (load) at every time instant. Due to the long response times in stack and BoP control, high temperature fuel cell processes can be inherently poorly suited for applications specifying rapid changes in stack power output. To overcome this obstacle, fuel cell systems having to respond to load changes can be equipped with massive energy storages and/or auxiliary energy dumping mechanisms to reduce the rate of change of operating conditions. In grid-tied systems designed for steady state operation, the inclusion of such equipment to be used only in case of grid failures (fault ride through capability) can be unbeneficial in terms of added value with respect to added cost, weight and system complexity.